Jillian Michaels is famous for her super-fit physique — but even she's not immune to the occasional body insecurity. Chatting with Wendy Williams on the Tuesday, Jan. 21, episode of The Wendy Williams Show, the Biggest Loser trainer admitted she's not entirely happy with her butt.

"I've never fully loved my booty," she confessed when asked which part of her figure she'd like to change. "It fits in the clothes nicely. In clothes, I can stuff that sucker into a pair of jeans and you would never know. But out of the jeans it just kinda…falls. And the texture makes me unhappy."

"Yes, it is!" Michaels — who turns 40 on Feb. 18 — replied. "I appreciate that it's like — there's good enough, and there's acceptance and everything — but for 40, I really want to give myself the best butt I can possibly have." The NBC star added that she'd never consider a butt implant and is instead trying butt-enhancing lunge exercises. "I'm trying to lunge it," she explained, "because I've never seen a bought booty that I find that impressive."

"My goal for 40 is to get out of bed and not walk away backwards. And Heidi's told me, she's like, 'I've seen your butt,'" the fitness guru continued, referring to partner Heidi Rhoades, with whom she has two kids, Lukensia and Phoenix. "I'm like, 'No, not really. Not in the bright light.'"

Michaels also spoke with Williams about dreading her upcoming birthday. "It's such a powerful number," she said of 40. "'Cause I'm going, 'I'm middle aged,' 'I'm middle aged!' I'm really trying to, like, breathe and embrace it. But it is kind of shocking, 'cause I don't think of myself as middle aged."